There's nothing mystical about piston tanks- just a big syringe, as you take on water the air compresses behind the piston, when you empty the tank, the water is pushed out, and the air pressure normalises.

Mike's subworks does endcaps up to 4"

Here in the UK, there are people who do cast endcaps upto 11 cm, which is over 4"-

You can get Lexan tubing from McMaster Carr in the States. In the UK, Lexan tubing is much more expensive than in the States, so we tend to use PMMA/Acrylic or PVC pipe. The latter is cheap (drain or waste pipe), and just as tough as Lexan, but is usually filled, so opaque. As a lot of people prefer a see-through dive module acrylic cylinders tend to be more popular. Acrylic is a lot more brittle than lexan or PVC, but is more rigid and less easily scuffed, and strong enough provided you don't go dropping it on hard surfaces etc. Tubing used here in the UK will be metric based, whilst U.S will be imperial.

I would add that the air pressure that builds up 'behind the piston' is actually building up within the dive module, so you will almost certainly want to ensure that the end caps are held in place with more than just the o-ring seals.

Ron Perrott has a neat way of holding on the endcaps on his piston tank equipped dive modules. The saddles that the cylinder sit on are designed in such a way that they overlap the endcaps at each end, and as they're glassed into the hull, they keep the endcaps pegged in place once the cylinder is strapped in, so no rods required.

Here's one of Ron's moudles installed in a 1/72nd scale Alfa class. Twin piston tanks fore and aft, and they self level.

So, an Engel tank fits inside a 4 inch wtc just barely. Interesting. I like doing business with Master Carr. Good source. Just so I am clear, if I have some 4 inch endcaps made up, we are talking a reccomended inside diameter of what, lexan tube how thick, which would give us the outside diameter. I want to be sure those end caps fit. Engel seems to be a runaway winner here for piston ballast systems. Experiance rules. How about one tank boats vs two tank boats. Say something around 70 inches or larger. Opinions here? advantages and disadvantages (besides cost and complexity-obvious considerations) Do we really buy them from Germany, or is there a dealer stateside?

Ideally, you want to use a two-piston setup. If you only use a single piston, the boat will submerge and surface with an angle. This is because with a single piston you are filling the 'ballast tank' from one end. E.g. if the piston is oriented so that the fill office is pointed towards the bow, the boat will submerge bow down and surface stern high. This assumes a completely static dive...i.e. no forward motion. Using two pistons with them pointed in opposite directions, you would get a nice level transition up or down.

I would suggest pulling the Engel piston specification sheet from the Engel web site and use that dimensioned drawing to verify that it fits inside a 4-inch tube. I just did a quick check with an Engel piston and a piece of 110mm tube...4-inches internal diameter will work.

As for wall thickness, a 1/8th-inch wall is more than thick enough. There will be some overpressure as you dive, but the deeper you go the more the external pressure will offset the internal pressure. On my Seehund, I calculate that once I get down to about 4 feet the pressures are about equal.

It's difficult to answer your question regarding a boat 70-inches or larger. It depends on two things: how much ballast you need to shift in order to go from a surface trim to a submerged trim and how large the Engel piston(s) are (they come in different sizes).

I am not aware of a current North American dealer for Engel. There used to be one, but that outfit is no longer s dealer.

At four feet you're only looking at about 2 psi, but in an earlier post you said your module compresses to 1.5bar (22psi) with the tank full. By my reckoning the boat would have to descend to beyond 40 feet before pressures equalized.

Does the C.G shift noticeably in your seehund? I have to say the shifting c.g. is very subtle in the boats I've seen equipped with single tanks, and when I've observed real boats diving they tend to take a bow down trim anyway.

If you want the ability to trim the c.g of your boat, an alternative is coupling a servo to a sliding weight, e.g. the main battery pack. Cheaper than a second tank and works well. This is well covered in Norberts book.

Engel aren't the only game in town when it comes to piston tanks. Piranha Modellbau also supply tanks to a slightly different design, and they come in small or large sizes. Ron Perrott was also making some new dive modules with twin tanks, but I'm not sure if these have moved beyond the beta testing phase.

At 1.5 bar there is an overpressure of 1/2 atmosphere when the piston is moved to the full dive position when on the workbench. This assumes that the dive module starts out with the internal pressure equalized to the surrounding atmosphere. So I would expect that at 16.5 feet it would be at basic equilibrium if the piston was at the full dive position. (33 feet = 1 atmosphere of additional pressure)

What I did not mention(!) is that to get to periscope depth, I do not need to completely fill the piston. But my math was still slightly off. Instead of 4 feet, I should have said 8 feet as I typically run with the piston in the half full position.

Yes, there is a noticeable change in trim during a completely static dive. As I stated, bow down when diving and stern high when surfacing. However while moving there is very little noticeable change in trim.

I investigated the sliding trim weight and decided that the added complexity wasn't worth the effort (in this instance).

I see where you coming from now, you were including atmospheric pressure in your figure. I thought you meant the air in the module was compressed 1.5bar above atmospheric, which I thought was a lot, and was surprised you didn't have seal problems.

The over forty feet part, came from the 1.5bar figure, if you compressed to that figure above atmospheric, then you would have to go that deep before the water pressure became greater.

As you are only half filling the tank, then the c.g would be behind the ballast once the boat was submerged. Most model I've seen the tank fills about 80-85% of the volume to submerge, so the difference in weight balance is less.

Here's Wheelerdealer's SS Akula with a 500ml engel tank fitted, goes up and down pretty much level.

Gail made up some 4" endcap masters and molds for his Type XXI WTC. When he sold his stuff Don Baker bought the molds. He might even have some endcaps -- you now how Gail loves to make extra. I'd contact him and see what he has.